Observing the solar corona from a formation-flying mission. First results of Proba-3/ASPIICS

TL;DR

Proba-3/ASPIICS demonstrates that two-spacecraft formation flying can deliver eclipse-like coronagraphy of the inner solar corona from about $1.099 R_ot$ out to $3 R_ot$, with very low straylight and high dynamic range. The mission's HDR imaging, polarization diagnostics, and multi-band capabilities enable imaging of quiescent structures, CMEs, and frequent small-scale outflows and inflows, revealing fine-scale solar wind formation processes previously unresolved. Key contributions include measurements of coronal polarization, density via $pB$ inversions, and the kinematics of CMEs between $1.5$ and $3 R_ot$, plus a detailed census of fast, small-scale dynamical features in the slow solar wind region that may be linked to S-web activity. Together, these results establish a new observational window for the inner corona, improving our understanding of coronal dynamics and the origins of the slow solar wind, with implications for space weather forecasting.

Abstract

We report the first results from observations of the solar corona by the ASPIICS coronagraph aboard the Proba-3 mission. ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is a giant coronagraph consisting of the telescope mounted aboard one of the mission's spacecraft and the external occulter placed on the second spacecraft. The two spacecraft separated by around 144 m fly in a precise formation up to 5.5 hours at a time, which allows coronal observations in eclipse-like conditions, i.e. close to the limb (typically down to 1.099 Rs, occasionally down to 1.05 Rs) and with very low straylight. ASPIICS observes quasi-stationary structures, such as coronal loops, streamers, quiescent prominences, and a variety of dynamic phenomena: erupting prominences, coronal mass ejections, jets, slow solar wind outflows, coronal inflows. In particular, weak, widespread and persistent small-scale outflows and inflows between 1.3 and 3 Rs are observed at a high spatial (5.6 arcsec) and temporal (30 s) resolution for the first time, expanding the range of scales at which the variable slow solar wind is observed to form.

Figures (12)

• Figure 1: Left panel: the solar K-corona observed by Proba-3/ASPIICS on 23 May 2025 at 17:50:31 UTC (orbit 207) in the wide (about 300 Å) spectral passband centered at 5510 Å. This high dynamic range image is composed of three images taken with exposure times of 0.1 s, 1 s, and 10 s. The internal occulter center is plotted as a gray cross. The position of the solar center and the solar disk are shown as the yellow cross and circle, respectively. Three dotted lines mark the directions along which the brightness is plotted in the right panel: dark region with seemingly open magnetic field (blue), bright streamer (red), and post-CME streamer (yellow). Three dots in the middle are the LEDs of the Occulter Position Sensor Emitter Zhukov2025, additional weak structures in the middle are due to the light reflections on the backside of the occulter. In all the images shown in this Letter solar north is up, west is to the right. Right panel: profiles of the K-corona brightness along the dotted lines indicated in the left panel, plotted as a function of radial distance in the corresponding color. The brightness turnover around 1.1 $R_\odot$ is due to the instrumental vignetting. The dashed and dotted lines show the typical intensities of the K-corona during solar maximum and F-corona, respectively Cox2000. The thin solid line shows the diffracted light intensity calculated using the model by Shestov2018.
• Figure 2: Left panel: a quiescent prominence observed by SDO/AIA in the He II passband (304 Å) on 25 March 2025 at 19:23:29 UTC. Right panel: a co-aligned image of the same prominence taken by ASPIICS in the He I D$_3$ passband (5876 Å) on 25 March 2025 at 19:23:23 UTC (orbit 135). In both panels, the solar coordinate grid is shown as well as the nominal inner edge of the ASPIICS field of view at 1.099 $R_\odot$.
• Figure 3: A CME observed by Proba-3/ASPIICS on 16 June 2025 (orbit 236). Left panel: He I D$_3$ (5876 Å) image of the corresponding erupting prominence taken at 07:27:41 UTC. The prominence twist is clearly visible. Right panel: Fe XIV (5303 Å) image of the CME taken at 07:38:47 UTC, with the image taken at 07:28:58 UTC subtracted. The CME bright front, cavity, and the erupting prominence can be seen. A radial filter was applied to enhance the visibility of the weak bright front. The associated movie (Movie 1) is available online.
• Figure 4: Kinematics of a CME observed by Proba-3/ASPIICS on 16 June 2025 (orbit 236), GOES/SUVI, and SOHO/LASCO C2 and C3. Top and bottom panels show the evolution of the height and radial velocity. Asterisks and squares show the propagation of the CME core and the leading edge (LE) respectively. A linear plus power law fit for the CME core propagation is shown as a purple line. A quadratic fit for the leading edge propagation is shown as a blue line. The velocity was calculated using the three-point differentiation.
• Figure 5: Distance-time map of the radial propagation of small-scale structures at the position angle of 204$^\circ$. The coronal brightness was averaged over the 2$^\circ$-wide sector (see Figure \ref{['fig:track']}). An image taken 5 minutes earlier was subtracted from every image. The cadence of the data was 30 s.